A common logistical concern in businesses is the tracking of assets or persons. In retail, one example of this logistical tracking concern is shoplifting. Many retail establishments employ electronic tags attached to goods that can be detected by systems installed for that purpose. A common term for these systems, tags, etc. is electronic article surveillance, or EAS.
Many of these tags and systems are only capable of registering the presence of the tag. Transmitters and receivers are located at exit points within a retail environment and the transmitter creates an interrogation zone at the exits while the receivers scan for responses from tags passing through the interrogation zone. The transmitters and receivers are typically housed in pedestals. There are several types of tags for these systems, one of which is a harmonic tag and another of which is a resonance tag. With the harmonic tag the electromagnetic interrogation field creates stored energy in the harmonic tag, and when the interrogation field is turned off this energy dissipates from the tag and produces a signal which is a harmonic of the interrogation field. With the resonant tags, the resonant tags vibrate with the interrogation field and produce a signal from this harmonic resonation. The system is tuned to the expected frequencies whether they are harmonic tags or resonant tags, and the receiver antennas of the system detect these signals. A common frequency for these systems is 58 kHz. When a signal is detected within an interrogation field, it is assumed that a tag is present and that it is improperly being removed from the retail facility. Similar systems may also be used to identify authorized personnel.
Generally, premises using electronic article surveillance systems have other electrical systems within them. These electrical systems will themselves put out electromagnetic waves, which are noise in the areas being monitored by the electronic article surveillance systems. More recently, developments in electronics in these premises have increased the level of noise in the specific frequency range in which the EAS systems typically operate. In an effort to achieve energy savings, many locations have changed to DC powered systems requiring DC to DC power supplies, resulting in the increased noise level in the pertinent frequency range. This increased level of noise in the operating frequency for the EAS systems makes it much more difficult to detect EAS tags in interrogation zones, since the strength of the signals from EAS tags is relatively weak. The present invention addresses this problem of high noise environments in EAS systems.
Another problem commonly encountered in electronic article surveillance systems is alarms generated by EAS tags not in the interrogation zone but near the pedestals housing the transmitters and receivers. The terminology employed with respect to this problem is “front field” and “back field”. The front field is in the interrogation zone, while the back field is generally on the opposite side of the antennas from the interrogation zone. The front of retail stores is frequently used as display areas for merchandise. Since exits are typically also located at the front of stores, this puts merchandise in close proximity to the EAS pedestals and enclosed antennas. It is preferred that the merchandise retain the EAS tags while on display, but the close proximity of tagged merchandise in the back field of the antennas can lead to false alarms. At least one embodiment of the present invention addresses this problem of alarms generated by EAS tags in the back field.
Many prior art EAS antenna systems have attempted to compensate for the presence of ambient noise in the retail environment. For an EAS monitoring system, the problem is compounded by the fact that the amplitude of the total noise from all sources, or the “composite noise envelope” may be greater than that of a single EAS tag that enters the interrogation zone. Moreover, noise sources are constantly changing in a dynamic environment in often unpredictable ways. Many of the prior art systems, including those discussed above, have relied upon digital filtering of noise using DSP to differentiate or “filter” noise from the EAS received signal. However, DSP-based filtering is not optimum because of the time delay associated with such systems. In addition, such systems are often limited in their effectiveness if they do not effectively model the composite noise environment prior to analyzing incremental changes that may (or may not) suggest the presence of a tag signal.